Acids, Bases And Salts

Hello students, welcome to today's science class. I'm so happy to see you all here, ready to learn something new and exciting. Today, we are going to study a chapter that is extremely important not just for your exams, but also for your everyday life. We are going to learn about Acids, Bases and Salts – Chapter 2 of your Science textbook.

Now, students, you must have tasted many things in your life. Think about this – what happens when you eat a lemon? You feel sour, right? And what about soap? When you accidentally get soap on your tongue while bathing, it tastes bitter. So, students, the sour taste in our food comes from acids, and the bitter taste comes from bases. You have already learned this in your previous classes, but today we will go much deeper into understanding why this happens and what these substances really are.

Now, let me ask you an interesting question. Suppose someone in your family is suffering from acidity after overeating. What would you suggest as a remedy – lemon juice, vinegar, or baking soda solution? Think about it for a moment. Which one would you choose? Of course, you would suggest baking soda solution, right? Why? Because you know that baking soda can neutralize the excess acid in the stomach. This is exactly what we are going to learn in this chapter – how acids and bases cancel out each other's effects.

Now, students, how do we know if a substance is an acid or a base without tasting it? We use something called indicators. You must have heard of litmus. Litmus is a purple dye that is extracted from a plant called lichen, which belongs to a group of plants called Thallophyta. When we put blue litmus paper in an acid, it turns red. When we put red litmus paper in a base, it turns blue. This is how we can identify acids and bases. There are other natural indicators too, like turmeric. Have you ever noticed that when you get curry stain on a white cloth and then scrub soap on it, the stain becomes reddish-brown? And when you wash it with plenty of water, it turns yellow again? This is because soap is basic in nature, and turmeric acts as an indicator. There are also synthetic indicators like methyl orange and phenolphthalein, which change colour in acidic or basic solutions.

Now, let me ask you a question. You have been provided with three test tubes. One of them contains distilled water, and the other two contain an acidic solution and a basic solution, respectively. If you are given only red litmus paper, how will you identify the contents of each test tube? This is an important question, so think carefully. You have only red litmus paper. Now, remember what red litmus does – it turns blue in a basic solution. But what happens in an acidic solution or in water? Red litmus remains red in acidic solution and also in water. So, here's what you can do – take the red litmus paper and dip it in each test tube. The test tube where the red litmus turns blue definitely contains a base. Now, you have two test tubes left – one with water and one with acid. How do you distinguish between them? You can take the blue litmus paper now. Remember, blue litmus turns red in acid but remains blue in water. So, take the blue litmus paper and dip it in the remaining two test tubes. The one where blue litmus turns red contains acid, and the one where it remains blue contains distilled water. Simple, isn't it?

Now, students, let's move on to understanding the chemical properties of acids and bases. This is where things get really interesting.

In the laboratory, we work with many acids and bases. Let me list some of them for you. Acids include hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃), and acetic acid (CH₃COOH). Bases include sodium hydroxide (NaOH), calcium hydroxide [Ca(OH)₂], potassium hydroxide (KOH), magnesium hydroxide [Mg(OH)₂], and ammonium hydroxide (NH₄OH). When we test these with indicators like red litmus, blue litmus, phenolphthalein, and methyl orange, we get different colours. For acids, red litmus stays red, blue litmus turns red, phenolphthalein remains colourless, and methyl orange turns red. For bases, red litmus turns blue, blue litmus stays blue, phenolphthalein turns pink, and methyl orange turns yellow. This is how we can identify whether a solution is acidic or basic.

Now, students, there is something very interesting called olfactory indicators. These are substances whose smell changes in acidic or basic media. Let me tell you about an activity you can do at home. Take some finely chopped onions in a plastic bag along with some clean cloth strips. Tie the bag tightly and leave it overnight in the fridge. The cloth strips will absorb the onion smell. Now, take two of these cloth strips and check their smell. Then, put a few drops of dilute hydrochloric acid on one strip and a few drops of dilute sodium hydroxide on the other. Rinse both strips with water and check their smell again. You will notice that the smell of onion disappears or changes when treated with acid or base. Similarly, you can test vanilla essence and clove oil. When you add dilute hydrochloric acid or dilute sodium hydroxide to vanilla essence or clove oil, their smell changes. So, vanilla, onion, and clove can all be used as olfactory indicators. Isn't that fascinating?

Now, let's do some more activities to understand how acids and bases react with other substances. First, let's see how acids react with metals. This is a very important reaction, so pay attention.

Take about 5 millilitres of dilute sulphuric acid in a test tube and add a few pieces of zinc granules to it. What do you observe? You will see that bubbles form on the surface of the zinc granules. This gas is hydrogen gas. Now, how do we confirm that it is hydrogen? We pass this gas through soap solution, and bubbles form. When we bring a burning candle near these bubbles, we hear a popping sound. This pop is the characteristic test for hydrogen gas. Now, students, this reaction can be written as:

Sulphuric acid + Zinc → Zinc sulphate + Hydrogen gas

In chemical notation, it is:

H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)

Now, let's repeat this activity with other acids like hydrochloric acid (HCl), nitric acid (HNO₃), and acetic acid (CH₃COOH). Are the observations the same or different? With hydrochloric acid and acetic acid, we also get hydrogen gas. But with nitric acid, the reaction is different because nitric acid is a strong oxidizing agent. So, students, remember that not all acids produce hydrogen gas when reacted with metals. But generally, we can say that acid + metal gives salt + hydrogen gas.

Now, what happens when we react a base with a metal? Let's try this activity. Take some granulated zinc metal in a test tube and add 2 millilitres of sodium hydroxide solution. Warm the contents of the test tube. What do you observe? You will see that hydrogen gas is evolved here too. The reaction is:

2NaOH(aq) + Zn(s) → Na₂ZnO₂(s) + H₂(g)

This produces sodium zincate and hydrogen gas. So, students, some bases also react with metals to produce hydrogen gas. However, not all metals react with bases in this manner.

Now, let's see how metal carbonates and metal hydrogencarbonates react with acids. This is also very important. Take two test tubes. In test tube A, take about 0.5 grams of sodium carbonate (Na₂CO₃). In test tube B, take about 0.5 grams of sodium hydrogencarbonate (NaHCO₃). Now, add about 2 millilitres of dilute hydrochloric acid to both test tubes. What do you observe? You will see that bubbles of gas are produced in both test tubes. This gas is carbon dioxide. Now, pass this gas through lime water, which is calcium hydroxide solution. The lime water turns milky. This confirms that the gas is carbon dioxide. The reactions are:

For sodium carbonate: Na₂CO₃(s) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + CO₂(g)

For sodium hydrogencarbonate: NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

When we pass carbon dioxide through lime water: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

The white precipitate of calcium carbonate makes the lime water look milky. If we pass excess carbon dioxide, the milkiness disappears because calcium carbonate reacts with carbon dioxide and water to form calcium hydrogencarbonate, which is soluble in water:

CaCO₃(s) + H₂O(l) + CO₂(g) → Ca(HCO₃)₂(aq)

Students, limestone, chalk, and marble are all different forms of calcium carbonate. So, all metal carbonates and hydrogencarbonates react with acids to give a corresponding salt, carbon dioxide, and water. This is a very useful reaction.

Now, let's see what happens when acids and bases react with each other. This is called a neutralisation reaction. Take about 2 millilitres of dilute sodium hydroxide solution in a test tube and add two drops of phenolphthalein solution. What colour do you see? Phenolphthalein is colourless in acidic solution but turns pink in basic solution. Since sodium hydroxide is a base, the solution will turn pink. Now, add dilute hydrochloric acid to this solution drop by drop. What happens? The pink colour disappears! Why did this happen? Because the acid neutralized the base. Now, if you add a few more drops of sodium hydroxide to this mixture, the pink colour reappears. This shows that the reaction between an acid and a base is reversible to some extent, but essentially, they neutralize each other. The reaction is:

NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

This is a neutralisation reaction. In general, we can write: Base + Acid → Salt + Water

Now, students, let's see how metallic oxides react with acids. Take a small amount of copper oxide in a beaker and add dilute hydrochloric acid slowly while stirring. What do you observe? The colour of the solution becomes blue-green, and the copper oxide dissolves. The blue-green colour is due to the formation of copper(II) chloride. The reaction is:

CuO(s) + 2HCl(aq) → CuCl₂(aq) + H₂O(l)

Now, students, since metallic oxides react with acids to give salts and water, similar to how bases react with acids, metallic oxides are said to be basic oxides.

Now, what about non-metallic oxides? You saw the reaction between carbon dioxide and calcium hydroxide (lime water) in the previous activity. Calcium hydroxide, which is a base, reacts with carbon dioxide to produce a salt (calcium carbonate) and water. Since this is similar to the reaction between a base and an acid, we can conclude that non-metallic oxides are acidic in nature. For example, carbon dioxide (CO₂), sulphur dioxide (SO₂), and nitrogen dioxide (NO₂) are all acidic oxides.

Now, let me ask you some important questions. First, why should curd and sour substances not be kept in brass and copper vessels? This is because curd and sour substances contain acids. When acids come in contact with metals like copper or brass (which is an alloy of copper and zinc), they react to form toxic compounds. This can contaminate the food and make it harmful to eat. So, always store sour foods in glass or stainless steel containers.

Second, which gas is usually liberated when an acid reacts with a metal? The gas is hydrogen. For example, when zinc reacts with sulphuric acid, we get zinc sulphate and hydrogen gas. How do we test for hydrogen? We bring a burning candle near the gas. If it burns with a popping sound, it is hydrogen. The popping sound is due to the explosion caused by hydrogen burning rapidly in air.

Third, a metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride. Now, students, think about this. The gas that extinguishes a burning candle is carbon dioxide. So, compound A must be a carbonate or a hydrogencarbonate. Since one of the products is calcium chloride, compound A must be calcium carbonate (CaCO₃) or calcium hydrogencarbonate (Ca(HCO₃)₂). The reaction with hydrochloric acid would be:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Or

Ca(HCO₃)₂(aq) + 2HCl(aq) → CaCl₂(aq) + 2H₂O(l) + 2CO₂(g)

Both reactions produce carbon dioxide gas, which extinguishes a burning candle. Since the question mentions "a metal compound A" and one product is calcium chloride, the most likely compound is calcium carbonate.

Now, students, let's move on to a very important question. What do all acids have in common, and what do all bases have in common? We have seen that all acids have similar chemical properties. What leads to this similarity? Let's do an activity to find out.

Take solutions of glucose, alcohol, hydrochloric acid, sulphuric acid, and so on. Fix two nails on a cork and place the cork in a 100 millilitre beaker. Connect the nails to the two terminals of a 6-volt battery through a bulb and a switch. Now, pour some dilute hydrochloric acid in the beaker and switch on the current. What do you observe? The bulb glows! This means that electricity is flowing through the solution. Now, repeat with dilute sulphuric acid. The bulb glows again. But when you repeat the experiment with glucose and alcohol solutions, the bulb does not glow. This is very important, students. The glowing of the bulb indicates that there is a flow of electric current through the solution. The electric current is carried through the acidic solution by ions. So, acids must be producing ions in solution.

Now, what ions do acids produce? Acids contain hydrogen as the cation. In hydrochloric acid (HCl), we have H⁺ and Cl⁻ ions. In nitric acid (HNO₃), we have H⁺ and NO₃⁻ ions. In sulphuric acid (H₂SO₄), we have H⁺ and SO₄²⁻ ions. In acetic acid (CH₃COOH), we have H⁺ and CH₃COO⁻ ions. So, all acids produce hydrogen ions, H⁺(aq), in solution. This is what makes them acidic. Glucose and alcohol contain hydrogen, but they do not produce H⁺ ions in solution, so they do not show acidic properties. This is why compounds like alcohol and glucose, even though they contain hydrogen, are not classified as acids.

Now, let's see what happens when we test bases in the same way. If we take sodium hydroxide, calcium hydroxide, and so on, we will find that they also conduct electricity. This is because bases produce hydroxide ions, OH⁻(aq), in solution. So, students, this is the key point: all acids produce H⁺(aq) ions in solution, and all bases produce OH⁻(aq) ions in solution.

Now, an important question: do acids produce ions only in aqueous solution? Let's test this. Take about 1 gram of solid sodium chloride in a clean, dry test tube. Add some concentrated sulphuric acid to the test tube. What do you observe? We get hydrogen chloride gas. Now, test this gas with dry and wet blue litmus paper. The dry blue litmus paper does not change colour, but the wet blue litmus paper turns red. This shows that hydrogen chloride gas does not show acidic properties when it is dry, but when it dissolves in water (becoming wet), it shows acidic properties. This is because hydrogen ions are produced only in the presence of water. The reaction is:

HCl + H₂O → H₃O⁺ + Cl⁻

Hydrogen ions cannot exist alone; they combine with water molecules to form hydronium ions, H₃O⁺. So, we should always write H⁺(aq) or H₃O⁺ to represent hydrogen ions in solution.

Similarly, when bases dissolve in water: NaOH(s) →[H₂O] Na⁺(aq) + OH⁻(aq)

KOH(s) →[H₂O] K⁺(aq) + OH⁻(aq)

Mg(OH)₂(s) →[H₂O] Mg²⁺(aq) + 2OH⁻(aq)

Bases generate hydroxide ions in water. Bases that are soluble in water are called alkalis. For example, sodium hydroxide and potassium hydroxide are alkalis, but calcium hydroxide is not very soluble in water, so it is not an alkali.

Now, students, let's understand what happens in a neutralisation reaction at the ionic level. We have: Acid + Base → Salt + Water

In terms of ions, this is: H⁺(aq) + OH⁻(aq) → H₂O(l)

This is the net ionic equation for all neutralisation reactions.

Now, let's talk about an important practical aspect. When we mix an acid or a base with water, what happens? Take 10 millilitres of water in a beaker. Add a few drops of concentrated sulphuric acid to it and swirl the beaker slowly. Touch the base of the beaker. Is there a change in temperature? Yes, the beaker becomes hot. This means the process is exothermic – it releases heat. The same happens when we dissolve sodium hydroxide pellets in water. So, students, the process of dissolving an acid or a base in water is highly exothermic. This is very important to remember. When diluting an acid, we must always add the acid to water, not water to the acid. If we add water to concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating. So, always remember: add acid to water, not water to acid. And stir constantly while adding.

Now, let's move on to understanding how strong acid or base solutions are. We know how to use indicators to distinguish between acids and bases, but can we measure how strong an acid or a base is? Yes, we can, using the pH scale.

A universal indicator is a mixture of several indicators that shows different colours at different concentrations of hydrogen ions. The pH scale has been developed to measure the hydrogen ion concentration. The "p" in pH stands for "potenz" in German, meaning power. On the pH scale, we measure from 0 to 14. A pH of 7 is neutral. pH less than 7 is acidic, and pH greater than 7 is basic. Higher the hydronium ion concentration, lower is the pH value.

Now, let's test the pH of some common substances. You can do this activity in your laboratory. Take solutions like saliva (before and after meal), lemon juice, colourless aerated drink, carrot juice, coffee, tomato juice, tap water, 1M sodium hydroxide, and 1M hydrochloric acid. Dip a pH paper in each solution and note the colour. Compare with the standard pH chart to find the approximate pH value. You will find that lemon juice has pH around 2, which is strongly acidic. Coffee has pH around 5, which is weakly acidic. Water has pH 7, which is neutral. Sodium hydroxide solution has pH around 14, which is strongly basic. This activity will help you understand the pH of various everyday substances.

Now, students, what is the difference between a strong acid and a weak acid? If we take hydrochloric acid and acetic acid of the same concentration, say one molar, they produce different amounts of hydrogen ions. Hydrochloric acid gives more H⁺ ions, so it is a strong acid. Acetic acid gives fewer H⁺ ions, so it is a weak acid. Similarly, strong bases give more OH⁻ ions, and weak bases give fewer OH⁻ ions.

Now, let's discuss the importance of pH in everyday life. Our body works within a pH range of 7.0 to 7.8. Living organisms can survive only in a narrow range of pH change. When the pH of rain water is less than 5.6, it is called acid rain. When acid rain flows into rivers, it lowers the pH of the river water, making it difficult for aquatic life to survive.

The pH of soil is also very important for plant growth. Different plants require different pH ranges for healthy growth. You can collect soil from various places, mix it with water, filter it, and check the pH of the filtrate using a universal indicator paper. This will help you understand which plants grow in which type of soil.

Our stomach produces hydrochloric acid, which helps in the digestion of food. During indigestion, the stomach produces too much acid, causing pain and irritation. To get rid of this pain, people use antacids, which are bases that neutralize the excess acid. Magnesium hydroxide, also known as milk of magnesia, is commonly used as an antacid.

Tooth decay is another important topic. Tooth decay starts when the pH of the mouth is lower than 5.5. Tooth enamel is made of calcium hydroxyapatite, which is the hardest substance in the body. It does not dissolve in water, but it gets corroded when the pH in the mouth is below 5.5. Bacteria in the mouth produce acids by degrading sugar and food particles remaining in the mouth after eating. The best way to prevent tooth decay is to clean the mouth after eating. Using toothpaste, which is generally basic, can neutralize the excess acid and prevent tooth decay.

Now, students, have you ever been stung by a honey bee? Bee sting leaves an acid that causes pain and irritation. Applying a mild base like baking soda on the stung area gives relief. Similarly, nettle leaves inject methanoic acid, causing a burning pain. A traditional remedy is to rub the area with the leaf of the dock plant, which often grows beside nettle. Can you guess the nature of the dock plant? It must be basic to neutralize the acid. So, next time you go trekking and accidentally touch a nettle plant, you know what to look for.

Now, let's look at some naturally occurring acids. Vinegar contains acetic acid, sour milk (curd) contains lactic acid, orange and lemon contain citric acid, tamarind contains tartaric acid, tomato contains oxalic acid, ant sting and nettle sting contain methanoic acid. These are all natural acids found in our food and in nature.

Now, let's answer some important questions. First, you have two solutions, A and B. The pH of solution A is 6, and the pH of solution B is 8. Which solution has more hydrogen ion concentration? Since pH 6 is less than pH 8, solution A has more hydrogen ion concentration. Solution A is acidic, and solution B is basic.

Second, what effect does the concentration of H⁺(aq) ions have on the nature of the solution? Higher the concentration of H⁺ ions, more acidic the solution. Lower the concentration of H⁺ ions, less acidic the solution.

Third, do basic solutions also have H⁺(aq) ions? If yes, then why are these basic? Yes, basic solutions also have H⁺ ions, but they have more OH⁻ ions than H⁺ ions. The presence of more OH⁻ ions makes the solution basic.

Fourth, under what soil condition would a farmer treat the soil with quick lime (calcium oxide) or slaked lime (calcium hydroxide) or chalk (calcium carbonate)? If the soil is acidic (pH less than 7), the farmer would add these basic substances to neutralize the acid and bring the pH to a suitable level for plant growth.

Now, let's move on to salts. Salts are compounds formed when acids react with bases. We have seen the formation of salts in various reactions. Let's learn more about them.

First, let's look at the family of salts. Write the chemical formulae of the following salts: potassium sulphate (K₂SO₄), sodium sulphate (Na₂SO₄), calcium sulphate (CaSO₄), magnesium sulphate (MgSO₄), copper sulphate (CuSO₄), sodium chloride (NaCl), sodium nitrate (NaNO₃), sodium carbonate (Na₂CO₃), and ammonium chloride (NH₄Cl). Notice the pattern in their names and formulae. Salts are named after the acid and base from which they are formed.

Now, let's check the pH of salts. Take samples of sodium chloride, potassium nitrate, aluminium chloride, zinc sulphate, copper sulphate, sodium acetate, sodium carbonate, and sodium hydrogencarbonate. Check their solubility in water. Then, test their action on litmus and find the pH using pH paper. What do you observe? Salts of a strong acid and a strong base are neutral, with pH 7. Salts of a strong acid and a weak base are acidic, with pH less than 7. Salts of a strong base and a weak acid are basic, with pH more than 7. For example, sodium chloride (from HCl and NaOH) is neutral. Aluminium chloride (from HCl and Al(OH)₃) is acidic. Sodium carbonate (from NaOH and H₂CO₃) is basic.

Now, let's talk about chemicals from common salt. You must have heard of sodium chloride – the salt we use in food. It is obtained from seawater or from deposits of rock salt. Rock salt was formed when seas of bygone ages dried up. It is mined like coal. You must have heard about Mahatma Gandhi's Dandi March, where salt became a symbol of India's struggle for freedom.

Now, common salt is a raw material for many useful chemicals. Let's see how we can make various substances from sodium chloride.

First, sodium hydroxide. When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide. This process is called the chlor-alkali process (chlor for chlorine, alkali for sodium hydroxide). The reaction is:

2NaCl(aq) + 2H₂O(l) → 2NaOH(aq) + Cl₂(g) + H₂(g)

Chlorine gas is given off at the anode, hydrogen gas at the cathode, and sodium hydroxide solution is formed near the cathode. All three products are useful.

Second, bleaching powder. Chlorine gas produced during electrolysis is used to make bleaching powder. Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)₂]. The reaction is:

2Ca(OH)₂ + 2Cl₂ → Ca(ClO)₂ + CaCl₂ + 2H₂O

Bleaching powder is used for bleaching cotton and linen in textile industry, for bleaching wood pulp in paper factories, for bleaching washed clothes in laundry, as an oxidising agent in chemical industries, and to make drinking water free from germs.

Third, baking soda. The chemical name of baking soda is sodium hydrogencarbonate (NaHCO₃). It is produced using sodium chloride as one of the raw materials. The reaction is:

NaCl + H₂O + CO₂ + NH₃ → NH₄Cl + NaHCO₃

Baking soda is a mild non-corrosive basic salt. When heated during cooking, it decomposes:

2NaHCO₃ →[Heat] Na₂CO₃ + H₂O + CO₂

Baking soda has many uses. It is used in baking powder, which is a mixture of baking soda and tartaric acid. When heated or mixed in water, carbon dioxide is produced, which makes bread or cake rise and become soft and spongy. Baking soda is also used in antacids to neutralize excess acid in the stomach. It is also used in soda-acid fire extinguishers.

Fourth, washing soda. Another chemical obtained from sodium chloride is sodium carbonate (Na₂CO₃.10H₂O), which is washing soda. It is obtained by heating baking soda and then recrystallising sodium carbonate. Washing soda is used in glass, soap, and paper industries, in the manufacture of sodium compounds like borax, as a cleaning agent for domestic purposes, and for removing permanent hardness of water.

Now, students, let's talk about water of crystallisation. Have you ever noticed that some salts have water molecules attached to them? For example, copper sulphate crystals are blue. When you heat them, they turn white. What happens? The water of crystallisation is removed. Let me explain this activity.

Heat a few crystals of copper sulphate in a dry boiling tube. What is the colour of copper sulphate after heating? It becomes white. Do you notice water droplets in the boiling tube? Yes, water droplets appear on the sides of the tube. These water molecules came from the copper sulphate crystals. Now, add 2-3 drops of water on the sample of copper sulphate obtained after heating. What do you observe? The blue colour of copper sulphate is restored! This is because copper sulphate crystals contain water of crystallisation – water molecules that are chemically bonded to the salt. The formula for hydrated copper sulphate is CuSO₄.5H₂O. It has five water molecules per formula unit.

Similarly, washing soda has 10 water molecules: Na₂CO₃.10H₂O. This is called decahydrate. Gypsum has two water molecules: CaSO₄.2H₂O.

Now, let's talk about Plaster of Paris. When gypsum is heated at 373 Kelvin, it loses water molecules and becomes calcium sulphate hemihydrate (CaSO₄.½H₂O). This is called Plaster of Paris. It is a white powder. When mixed with water, it changes back to gypsum, giving a hard solid mass. The reaction is:

CaSO₄.½H₂O + 1½H₂O → CaSO₄.2H₂O

Plaster of Paris is used for making toys, materials for decoration, and for making surfaces smooth. It is also used by doctors for supporting fractured bones. Plaster of Paris should be stored in a moisture-proof container because if it absorbs moisture, it will harden and become useless.

Now, let's answer some questions. What is the common name of the compound Ca(ClO)₂? It is bleaching powder. Name the substance which on treatment with chlorine yields bleaching powder. It is dry slaked lime [Ca(OH)₂]. Name the sodium compound which is used for softening hard water. It is washing soda (sodium carbonate). What will happen if a solution of sodium hydrogencarbonate is heated? It will decompose to form sodium carbonate, water, and carbon dioxide. The equation is:

2NaHCO₃ →[Heat] Na₂CO₃ + H₂O + CO₂

Write an equation to show the reaction between Plaster of Paris and water: CaSO₄.½H₂O + 1½H₂O → CaSO₄.2H₂O

Now, let's solve the exercise questions.

Question 1: A solution turns red litmus blue, its pH is likely to be (a) 1 (b) 4 (c) 5 (d) 10. Since it turns red litmus blue, it is basic. So, the pH must be more than 7. The answer is (d) 10.

Question 2: A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains (a) NaCl (b) HCl (c) LiCl (d) KCl. Egg-shells are made of calcium carbonate. When an acid reacts with calcium carbonate, carbon dioxide gas is produced, which turns lime-water milky. So, the solution must be an acid. Hydrochloric acid (HCl) is the correct answer. The answer is (b) HCl.

Question 3: 10 mL of a solution of NaOH is found to be completely neutralised by 8 mL of a given solution of HCl. If we take 20 mL of the same solution of NaOH, the amount HCl solution (the same solution as before) required to neutralise it will be (a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL. This is a question of proportion. If 10 mL of NaOH requires 8 mL of HCl, then 20 mL of NaOH will require 16 mL of HCl. The answer is (d) 16 mL.

Question 4: Which one of the following types of medicines is used for treating indigestion? (a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic. For indigestion, we use antacids to neutralize excess acid in the stomach. The answer is (c) Antacid.

Question 5: Write word equations and then balanced equations for the reaction taking place when: (a) dilute sulphuric acid reacts with zinc granules. Word equation: Sulphuric acid + Zinc → Zinc sulphate + Hydrogen Balanced equation: H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)

(b) dilute hydrochloric acid reacts with magnesium ribbon. Word equation: Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen Balanced equation: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g)

(c) dilute sulphuric acid reacts with aluminium powder. Word equation: Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen Balanced equation: 3H₂SO₄(aq) + 2Al(s) → Al₂(SO₄)₃(aq) + 3H₂(g)

(d) dilute hydrochloric acid reacts with iron filings. Word equation: Hydrochloric acid + Iron → Iron chloride + Hydrogen Balanced equation: 6HCl(aq) + 2Fe(s) → 2FeCl₃(aq) + 3H₂(g)

Question 6: Compounds such as alcohols and glucose also contain hydrogen but are not categorised as acids. Describe an Activity to prove it. We have already done this activity in class. Take solutions of glucose and alcohol. Connect them to a battery through a bulb, just like we did with acids. The bulb does not glow in the case of glucose and alcohol, which means they do not conduct electricity. This is because they do not produce hydrogen ions (H⁺) in solution. They contain hydrogen, but the hydrogen is covalently bonded and does not dissociate to form ions. Acids, on the other hand, produce H⁺ ions in solution and conduct electricity. So, this activity proves that alcohols and glucose are not acids, even though they contain hydrogen.

Question 7: Why does distilled water not conduct electricity, whereas rain water does? Distilled water is pure water with no dissolved ions. It does not contain any ions to conduct electricity. Rain water, on the other hand, dissolves carbon dioxide from the atmosphere and forms carbonic acid, which dissociates to produce hydrogen ions and carbonate ions. These ions conduct electricity. Also, rain water may pick up dust and other particles that can conduct electricity.

Question 8: Why does dry HCl gas not show acidic behaviour in the absence of water? This is because acids produce H⁺ ions only in the presence of water. Dry HCl gas does not have water molecules to dissociate into ions. When we test it with dry litmus paper, there is no colour change. But when we dissolve it in water, it produces H⁺ ions and shows acidic behaviour.

Question 9: Five solutions A, B, C, D and E when tested with universal indicator showed pH as 4, 1, 11, 7 and 9, respectively. Which solution is (a) neutral? (b) strongly alkaline? (c) strongly acidic? (d) weakly acidic? (e) weakly alkaline? Arrange the pH in increasing order of hydrogen-ion concentration. Solution with pH 7 is neutral, so solution D is neutral. pH 11 is strongly alkaline, so solution C is strongly alkaline. pH 1 is strongly acidic, so solution B is strongly acidic. pH 4 is weakly acidic, so solution A is weakly acidic. pH 9 is weakly alkaline, so solution E is weakly alkaline. Now, arrange in increasing order of hydrogen-ion concentration: pH 1 has the highest H⁺ concentration, then pH 4, then pH 7, then pH 9, then pH 11 has the lowest H⁺ concentration. So, the order is: B (1), A (4), D (7), E (9), C (11).

Question 10: Equal lengths of magnesium ribbons are taken in test tubes A and B. Hydrochloric acid (HCl) is added to test tube A, while acetic acid (CH₃COOH) is added to test tube B. Amount and concentration taken for both the acids are same. In which test tube will the fizzing occur more vigorously and why? Fizzing is due to the evolution of hydrogen gas. Hydrochloric acid is a strong acid, meaning it produces more H⁺ ions in solution. Acetic acid is a weak acid, meaning it produces fewer H⁺ ions. So, the reaction will be more vigorous in test tube A with hydrochloric acid. The fizzing will occur more vigorously in test tube A.

Question 11: Fresh milk has a pH of 6. How do you think the pH will change as it turns into curd? Explain your answer. As milk turns into curd, bacteria act on the lactose (milk sugar) and produce lactic acid. This increases the acidity of the milk. So, the pH will decrease from 6 to a lower value. The exact pH of curd depends on how much acid is produced, but it will be less than 6, making it more acidic.

Question 12: A milkman adds a very small amount of baking soda to fresh milk. (a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline? (b) Why does this milk take a long time to set as curd? (a) The milkman adds baking soda to make the milk slightly alkaline. This is because fresh milk is slightly acidic (pH 6), and in alkaline conditions, the growth of bacteria that cause curdling is slowed down. This helps the milk stay fresh for longer. (b) The milk takes a long time to set as curd because the bacteria that convert milk into curd thrive in acidic conditions. Since the milk is now alkaline, these bacteria cannot grow and multiply as quickly. So, it takes longer for the milk to turn into curd.

Question 13: Plaster of Paris should be stored in a moisture-proof container. Explain why? Plaster of Paris is calcium sulphate hemihydrate. It reacts with moisture (water) to form gypsum (calcium sulphate dihydrate). If Plaster of Paris absorbs moisture from the air, it will harden and become useless. So, it must be stored in a moisture-proof container.

Question 14: What is a neutralisation reaction? Give two examples. A neutralisation reaction is the reaction between an acid and a base to give a salt and water. Example 1: NaOH + HCl → NaCl + H₂O. Example 2: Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O.

Question 15: Give two important uses of washing soda and baking soda. Uses of washing soda: (i) It is used in glass, soap, and paper industries. (ii) It is used for removing permanent hardness of water. Uses of baking soda: (i) It is used in baking powder to make bread and cakes soft and spongy. (ii) It is used as an antacid to treat acidity.

Now, students, we have covered the entire chapter. Let me give you a summary of what we have learned today.

In this chapter, we learned about acids, bases, and salts. We started with indicators – substances that help us identify acids and bases. Litmus, turmeric, methyl orange, and phenolphthalein are all indicators. We learned that acids turn blue litmus red, and bases turn red litmus blue.

We then studied the chemical properties of acids and bases. Acids react with metals to produce hydrogen gas and a salt. Bases can also react with some metals to produce hydrogen gas. Metal carbonates and hydrogencarbonates react with acids to produce salt, carbon dioxide, and water. Acids and bases react with each other in a neutralisation reaction to produce salt and water. Metallic oxides are basic oxides, and non-metallic oxides are acidic oxides.

We learned that all acids produce hydrogen ions (H⁺) in solution, and all bases produce hydroxide ions (OH⁻) in solution. The strength of an acid or base depends on the number of ions it produces. We learned about the pH scale, which measures the hydrogen ion concentration. pH 7 is neutral, pH less than 7 is acidic, and pH more than 7 is basic.

We discussed the importance of pH in everyday life – in our body, in soil, in digestion, and in preventing tooth decay. We learned about salts, their pH, and how they are formed from strong acids and bases or weak acids and bases.

We also learned about chemicals obtained from common salt – sodium hydroxide, bleaching powder, baking soda, and washing soda. We learned about water of crystallisation and Plaster of Paris.

Finally, we solved all the exercise questions to reinforce our understanding.

Students, this is a very important chapter that will help you understand many chemical reactions in your daily life. Make sure you practice writing the chemical equations and understand the concepts well. Thank you for listening attentively. See you in the next class!